Class II major histocompatibility complex (MCH) gene products are membrane glycoproteins, expressed mainly on macrophages and B cells, which are co-recognized with antigen by helper T cells and related L3T4+ T cells. T cells distinguish the polymorphic forms of class II molecules in such a way that, in most cases, they corecognize only one allelic form of the product of a given class II locus. The structural basis of this restriction in corecognition function may depend on amino acids in the class II molecule which act as contact residues for antigen (desetopes) or for the T cell receptor (histotopes). In order to analyze this, structural mutants of I-Ak-bearing B cell-B lymphoma hybridomas have been selected. These mutants have altered capacity to present antigen to T cell hybridomas. ABeta genes have been cloned from a pair of sibling mutant lines and shown by DNA-mediated gene transfer to be responsible for the altered antigen-presenting phenotype. These mutant genes have a single nucleotide change leading the replacement of glutamic acid with lysine at position 67 in the Beta chain of the I-Ak molecule. These results demonstrate that even a single residue change can have profound effects on antigen-presenting phenotype and demonstrate that the hypervariable region around position 67 of the Beta chain is very important in the function of I-A class II molecules.